1. Field of the Invention
The present invention relates to power switching circuitry, which can be used in a variety of situations for generating an output power supply from a source power supply.
2. Description of the Prior Art
Power switching circuits can be used in a variety of applications, in order to generate an output power supply required for a target device. As one example, a power switching circuit can be used to generate an output power supply for an electric motor from a source power supply, such as may be provided for example by a battery.
One known type of power switching circuitry comprises a circuit board, a heat conductive substrate carrying power semiconductors, and a heat absorbing mounting structure disposed in heat transferring contact with the heat conductive substrate. U.S. Pat. No. 5,715,141 describes one such arrangement of power switching circuitry, which includes a clamping structure for pressing the circuit board and the heat conductive substrate towards each other as well as toward the mounting structure, and a number of tube elements disposed between the circuit board and the heat conductive substrate. The tube elements are comprised at least partly of an electrically conductive material, and the clamping structure comprises a plurality of screws extending through both the circuit board and the heat conductive substrate, and being secured to the mounting structure, with each of the screws extending substantively coaxially through a respective one of the tube elements.
Whilst such an arrangement provides an electrical connection between the circuit board and the heat conductive substrate, and serves to press the heat conductive substrate against the mounting structure, it does suffer from a number of disadvantages. Firstly, the screws forming the clamping structure need to be secured into the mounting structure, and this involves providing machined clearance holes in the heat conductive substrate to allow those screws to pass through the heat conductive substrate. In addition to increasing manufacturing costs, the presence of such holes also reduces the current carrying capability of the heat conductive substrate, and reduces the thermal transfer that can occur from components to the heat conductive substrate, particularly components in the region of such holes.
Further, the use of the screws in the manner set out in U.S. Pat. No. 5,715,141 can lead to a variation in clamping pressure over time which can affect the thermal transfer characteristics between the heat conductive substrate and the heat absorbing mounting structure. This is due not only to natural expansions and contractions that occur dependent on temperature, but also due to relaxing of the circuit board over time as it yields under the clamping pressure, thereby reducing the clamping force exerted by the screws.
Furthermore, the provision of the tube elements and coaxially placed screws leads to a relatively complex manufacturing process, which increases the cost of the power switching circuitry.